When it comes to predicting the result of Britainâ€™s EU membership referendum, the telephone surveys showing a lead for the â€œRemainâ€ side are probably closer to the mark, despite their flaws, according to a study.
Polls conducted online that suggest the race is too close to call and those done by phone are all missing important data, but the Internet polls are out by a wider margin, according to Matt Singh, co-author of the report released on Tuesday, who called last yearâ€™s general election correctly on his Number Cruncher Politics blog.
QuickTake Perils of Polling
According to Singh, the â€œblind spotâ€ is a failure to weight the results according to the social attitudes of interviewees. At present, companies usually weight their samples for age, gender and how people voted in the past.
â€œThe online samples have got too many socially conservative people, and the phone polls have got too many socially liberal people,â€ Singh said in an interview. â€œOn the referendum, the true picture is closer to the phone polls, but theyâ€™re both wrong.â€
The June 23 vote on a so-called Brexit is a worry for Britainâ€™s polling companies, who were humiliated by their failure to predict the election result. With different polling methods yielding different results for the public attitude to the EU, they face the prospect of a second disaster.
In 2015, the different mistakes of different polling methods meant they were all wrong in the same direction.
Ahead of the election, â€œthe phone polls had too many Labour voters, but the Labour voters they had were socially liberal,â€ Singh said. This meant they missed the flow of Labour voters to the anti-immigration U.K. Independence Party.
Online polls meanwhile missed out on socially liberal Conservative voters, so they over-estimated the flow of Tories to UKIP. Both ended up overstating Labour support and understating Conservative votes.
Working with polling by Populus Ltd., Singh and his co-author James Kanagasooriam, head of analytics for the company, estimate that online polls lean too far toward a â€œLeaveâ€ vote by about 3 percentage points because of the bias in their sampling.
Phone polls overstate support for â€œRemainâ€ by about 5 percentage points but a second factor then helps correct that error: phone polling discourages people from choosing â€Donâ€™t know.â€
Online polls make it easier for people to say they havenâ€™t decided how theyâ€™re going to vote than phone polling does. When these voters are pushed, they tend to break in favor of the status quo.